The results surprised Tashkin. "I wouldn't give [marijuana] a clean bill of health, but at least this study says if there is a risk, it's very small," he says. Still, he says, marijuana has been shown to suppress the immune system and may increase the risk of pneumonia.

Decades of research have shown that cigarette smoking dramatically increases the risk of certain cancers. But controversy surrounds the risk of smoking weed. A 1999 study of blood donors suggested a link between marijuana and head and neck cancer, but a larger study in 2004 found no such connection. Still, work in the lab suggests marijuana can be dangerous. For example, pot smoke contains more of some cancer-causing chemicals than cigarettes do, thanks to the filterless nature of joints.